1. Field of the Invention
The present invention relates to a measurement device and, more particularly, to an adjustable ultrasonic gas flow measurement device capable of adjusting the measuring range flexibly for the gas under test.
2. Description of Related Art
In the conventional gas measurement technology, an ultrasonic measurement device is used to perform measurement on a gas; the concentration, the flow and the molecular weight of the gas can be calculated from the phase shift difference of the ultrasonic signals passing through the gas. A conventional phase detection circuit is used to perform phase comparison for ultrasonic signals, and the obtained phase shift difference is used as an actual phase shift difference to perform subsequent calculations. Please refer to FIG. 1, which is a schematic diagram of a conventional ultrasonic measurement device. As shown, the ultrasonic measurement device includes a gas pipe 10, a transmitter 11, a receiver 12 and a micro-processing module 13. The gas pipe 10 has an inlet 101 and an outlet 102, which are respectively provided for gas to flow into and flow out the gas pipe 10. The transmitter 11 and the receiver 12 are located at two sides of the gas pipe 10 respectively. The micro-processing module 13 includes a clock generator 131 and a phase detection circuit 132. When a user wants to measure a gas, the gas pipe 10 is first filled with the gas to be measured. The clock generator 131 drives the transmitter 11 to transmit an ultrasonic signal into the gas pipe from one end of the gas pipe 10, and sends the ultrasonic signal to the phase detection circuit 132 simultaneously. The receiver 12 sends the ultrasonic signal received in the other end of the gas pipe 10 to the phase detection circuit 132. The phase detection circuit 132 compares the phase shifts of the transmitted ultrasonic and the received ultrasonic signals, and outputs a phase comparison result.
However, since the phase shift difference obtained from comparison is used as the actual phase shift difference in the above-mentioned measurement, an error may occur if the actual phase shift difference is greater than one phase. Please refer to FIG. 2, which is a schematic diagram of ultrasonic phase comparison of a conventional ultrasonic measurement device, wherein waveform denoted by (A) is the ultrasonic signal transmitted in the gas pipe, and waveforms denoted by (B1) and (B2) are the ultrasonic signals received from the pipe under two different measurement conditions. As shown in FIG. 2, a phase shift difference t1 can be obtained by comparing (A) and (B1), a phase shift difference t2 can be obtained by comparing (A) and (B2), but the actual phase shift difference of (B2) is t2, so that error may occur.
Therefore, when measuring a gas by using a conventional ultrasonic measurement device, the phase shift difference of ultrasonic signals passing through the gas should be smaller than one phase. However, the phase shift difference of ultrasonic signals changes with the molecular weight, temperature or the concentration of the gas under test. That is, the phase shift difference of ultrasonic signals passing through the gas under test is getting greater if the concentration of the gas is getting higher. When the phase shift difference of ultrasonic signals exceeds one phase, the measurement range of the conventional ultrasonic measurement device is exceeded. For example, the concentration of a gas under test increases, the flight time of sound will grow up, and the phase shift difference of ultrasonic signals may also increase with time, resulting in that, if using the conventional ultrasonic measurement device to measure the gas, error may occur when the phase shift difference reaches one phase. The same phenomenon was examined from flow measurement.
In the prior art, if it is desired to enlarge the measurement range of the ultrasonic measurement device, users may increase the size of the gas pipe or change the measurement resolution, which are inconvenient for users. Therefore, it is desirable to provide an ultrasonic measurement device to mitigate and/or obviate the aforementioned problems.